1. Field of the Invention
The present invention is directed generally to monitoring operating parameters in a steam turbine and, more specifically, to the monitoring of the vibration of rotating turbine blades.
2. Description of the Background
Turbine blades, because of their complex design, are subject to vibration at frequencies which correspond to natural frequencies of the blades. Each natural frequency is associated with a particular mode, each of which is a different combination of vibrational deflections such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, etc. To prevent excessive vibration of the blade about its normal position, prudent design practice dictates that the blades be constructed such that the frequencies of the lowest modes fall between harmonics of the operating frequency of the turbine. However, manufacturing tolerances, changes in blade attachment to the rotor, changes in blade geometry due to erosion, and changes in the operating frequency of the turbine, among other factors, may cause modal frequencies to approach the harmonics of the operating frequency.
The approach of a modal frequency to a harmonic of the operating frequency may result in vibration. Alternatively, the blades may be excited by non-synchronous forces associated with aerodynamic phenomena such as buffeting or flutter. That may occur even if the natural resonance frequencies of the blade are not near the harmonics of running speed of the turbine. When the amplitude of the vibration exceeds a certain level, objectionable stresses are set up in the blade. If the condition is not detected and remedied, the blade may eventually fracture resulting in an extremely costly forced outage of the machinery. Thus, a method for detecting vibration is useful to prevent such damage.
One prior art method for detecting turbine blade vibration uses permanently installed, non-contacting proximity sensors. An example of such an apparatus is disclosed in U.S. Pat. No. 4,573,358 to Luongo wherein a plurality of sensors spaced about the periphery of the blade row detect vibration of operator selected blades. With that apparatus, one blade can be monitored at any given time. Other apparatus utilizing non-contacting proximity sensors are U.S. Pat. No. 4,593,566 to Ellis and U.S. Pat. No. 4,887,468 to McKendree et al.
Digital turbine blade vibration data produced in accordance with the teachings of the aforementioned patents may include, in addition to the pertinent blade tip deflection data, blade tip deflection data common to all the blades in the row. That data is referred to as common mode vibration data because all of the blades experience the same vibration as a result thereof. Phenomena known to cause such common mode tip deflections are static displacement, torsional vibration, and rotor speed changes. If the frequencies of such displacement are low, it is known that the blade stresses as a result of such tip deflections are not significant because the blades are displaced as a whole, with little or no relative motion between various positions of a blade.
Current data analysis techniques used on blade tip deflection data to determine if common mode vibration data is present are performed manually and are comprised of the following steps:
finding the blade passing time data corresponding to the blade of interest;
computing the displacement by multiplying the difference between the actual blade passing times and the expected blade passing times by the turbine blade tip velocity;
Fourier-transforming the displacement data to obtain deflection as a function of harmonic; and
manually observing the data.
Upon manual observation, vibration that is seen to affect all blades about equally in amplitude and phase may be determined to be due to speed changes or to low frequency torsional responses. The operator must then exercise his judgment and either totally discard the data set or estimate and remove the contribution to the vibrational data from those effects. Clearly, such a process is very time consuming and calls the skill of the operator into play to determine when a tip deflection can be ignored. Analysis of apparent blade vibration data may actually prove to be nothing more than a measurement of a non-vibrational effect such as static displacement. Operator intervention to make such judgments and to select valid data is not practicable for an on-line blade vibration monitor. Thus, the need exists for an apparatus and method for removing common mode blade vibration data from data representative of other types of blade vibration.